Optical volume memory

ABSTRACT

An optical memory material includes a two-photon storage component which can be written from a first to a second state in response to WRITE light, mixed with a signal component which fluoresces by one-photon absorption only at the written locations in response to READ light. The storage material may be a fulgide. The memory material may also include a frequency upconversion material to aid writing. Writing is performed by a spatial light modulator (SLM) with a dynamic focussing system, for concentrating sufficient power at WRITE locations for nonlinear two-photon absorption. Crosstalk is avoided during simultaneous writing in some embodiments, by spacing the individual WRITE beams apart by an integer number of inter-beam spacings, so that non-adjacent datels are written simultaneously in a &#34;paragraph,&#34; and the non-written areas are written at a different time with different paragraphs. The memory material may be translated relative to the SLM to access different paragraphs, or accessed by an electronically sparsed SLM. Reading uses a sheet of READ light traversing the pages of written material to cause the signal component to fluoresce, and imaging the fluorescent pattern onto a detector array. The memory material may be stacked in layers, spaced apart by light waveguides, for guiding the READ beam to the page to be read. In another embodiment, writing and erasure are performed by a modulated quasi-one-dimensional sheet of light, intersecting a second, unmodulated sheet of light at a column within the memory material.

FIELD OF THE INVENTION

This invention relates to rewriteable data memories, and morespecifically to memories into which data may be written, and from whichthe recorded data may be read, by means of light, and in which the datais stored in the form of various states of the material of the memory.

BACKGROUND OF THE INVENTION

Data storage memories are widely used in computers and control systemsof various types. Computers and control processors ordinarily useelectronic random-access memories (RAMs) to aid in performing theirprocesses. Such electronic RAMs have the advantage of high operatingspeed, but their volume storage density is relatively low, and they arevolatile, in that the data stored therein is lost when the system isdeenergized. To save the data in a volatile RAM preparatory todeenergization, the data is ordinarily transferred to a rewriteablepermanent medium such as magnetic disc or magnetic tape. Disc and tapemedia are capable of storing large amounts of data, but have substantialinitial access time requirements to initially access or locate the data,and also have data transfer rates which are limited by the serial natureof the tape or the track on a disc. These different memory typesconstitute a hierarchy which lacks fast access time, high storagecapacity members.

Three-dimensional (3D) optical storage RAMs have been described, inwhich light beams address data elements (datels, also known as voxels)within the volume of the memory material, for writing data thereto, andfor reading. An article entitled Applications of Photochromic PolymerFilms, by A. E. J. Wilson, published at pp 232-238 of Volume 15, 1984issue of Phys. Technol., printed in Northern Ireland, describesphotochromic materials generally, their applications to optical datarecording, and also lists desirable aspects of an erasable reusableoptical recording medium, which include (1) high sensitivity for writingand erasing, (2) high storage capacity in bits per cm², (3)nondestructive readout, (4) lack of fatigue, which is the ability to becycled repeatedly without losing its characteristics, (5) archivalstorage or persistence of more than 10 years, (6) no requirement fordevelopment of the image, and (7) low cost and ease of fabrication.Canadian patent application 2,037,059, filed Feb. 26, 1991 in the nameof Daniels, and laid open Aug. 27, 1992, describes a system using liquidcrystals as the memory material, which are stained with a dye. A slightelectric field is applied across the memory. Writing is accomplished bya light beam, which heats the dye through which the beam passes, and theheat is transferred to the adjacent or contiguous liquid crystal datels,to allow them to change state under the influence of the electric field.In one embodiment of the Daniels memory, the heating is accomplished bymultiple intersecting beams of light. Patent Cooperation Treaty (PCT)patent application WO93/02454, filed in the name of Strickler, and laidopen Feb. 4, 1993, describes development of a three-dimensional opticalmemory in which a fluorescent dye is the storage medium, but which isundesirable because of photobleaching, and also describes an improvedthree-dimensional optical memory in which changes in the refractiveindex of a photopolymer are used for storage, and in which intersectingbeams of light are used to detect inhomogeneities (regions of alteredindex of refraction) in the medium. An article entitledThree-Dimensional Optical Storage Memory, authored by D. A.Parthenopoulos et al., and published at pp 843-845 of the Aug. 25, 1989issue of OE Reports, published by SPIE, the International Society forOptical Engineering, describes a three-dimensional optical memory basedon volume storage in an amplitude-recording medium, specifically thephotochromic molecule spirobenzopyran, which in a I (spiropyran) stateabsorbs visible light by a two-photon absorption process (simultaneousabsorption of two visible-light photons, corresponding to energy in theultraviolet or UV range), and when excited takes on a II (merocyanine)state. The I state may correspond to an unwritten (logic 0) state, sothat writing involves application of ultraviolet-energy light to createa II state in the datel region. The II state absorbs light in thegreen-red region of the visible light spectrum, and emits red-shiftedfluorescence when excited with green light. Thus, reading isaccomplished by applying a beam of green light to the datel, and the redshift identifies the written (logic 1) state. The persistence of the IIstate, however, ranges from a few minutes under ordinary conditions to afew weeks when cooled. An article entitled Potentials of two-photonbased 3-D optical memories for high performance computing, by Hunter etal., published at pp 2058-2066 of Applied Optics, Vol. 29, No.14, 10 May1990, also discusses the use of spirobenzopyran. The text ElectronicMaterials From Silicon to Organics, edited by L. S. Miller et al., andpublished by Plenum Publishing Corporation, 1991, includes at pp 471-483a chapter entitled Photochromics of the Future, authored by H. G.Heller, which notes that the main reason that organic photochromicmaterials have not been developed for commercial applications is theproblem of fatigue, and which describes the properties of fulgides andheliotropic compounds. An article entitled Two Photon Three DimensionalMemory Hierarchy, by S. Esner et al., presented at the July, 1992 SPIEmeeting at San Diego, describes the abovementioned hierarchy of memoriesfor use in computers and processors, and also describes two-photonsecondary storage (memory) systems which have the potential formillisecond access time and Tbit/sec data transfer rates, in whichspirobenzopyran material in a 3-dimensional memory is written byintersecting beams of light, and in which an HCl component of the memorymaterial provides permanent stability of the written form. However,permanent stability implies an inability to erase and re-write, or tooverwrite.

It should be noted that the abovementioned different colors of light areestablished by their wavelengths, which range in the visible spectrumfrom about 400 to 700 nanometers (nm), and it is also noted thatwavelength and frequency of light are inversely related by the velocityof propagation of light (C). The velocity of light is constant within aparticular medium, but different media exhibit different values of C.

Improved memories are desired.

SUMMARY OF THE INVENTION

A light-controlled memory stores data in the form of one of a pluralityof states of a multipartite memory material. Each bit of data is storedat a data element (datel) location, which may be at the surface of thememory material, or which may be within the bulk material. The memorymaterial is a combination of a storage component or material whichchanges state in response to a WRITE light and a readout or signalcomponent or material which provides an indication of the state of thestorage material at its location. The storage component may change statein response to two-photon absorption of the WRITE light, and the signalcomponent may respond by one-photon absorption of a READ light. Thememory material may also include a harmonic generation or frequencyup-conversion material which translates light wavelength for generatingthe WRITE light which is absorbed by the storage component of the memorymaterial. In a particular embodiment of the invention, the memorymaterial includes a mixture of a frequency up-conversion material suchas an "up-conversion" dye, a photochromic storage material, and afluorescent material such as a "signal" dye, which responds to the localstate of the adjacent photochromic material. In a more particularembodiment of the invention, the signal dye fluoresces upon illuminationonly when the storage material is in a first state, and does notfluoresce, or fluoresces only weakly, when illuminated when the storagematerial is in a second state. An up-conversion dye may be Coumarin 6.The storage material may be a fulgide. The fluorescent signal dye may beDODCI. An embodiment of the light-controlled memory includes a lightmodulator for modulating light beam(s) for writing into, and, whendesired, for erasing the memory material. The light modulator may be aspatial light modulator for modulating light beams to form atwo-dimensional representation of the data. The spatial light modulatormay be one-dimensional (1-D), quasi-one-dimensional (q-1-D), ortwo-dimensional (2 -D). A lens system focusses the light onto thedesired datel(s) at the surface of, or within, the memory material. Adynamic lens system, which may include a zoom lens, may be used to focuson various pages of memory within the body of the memory material. In aparticularly advantageous embodiment, focussing is accomplished by alens system including a microlens array, for simultaneously accessingplural, spaced-apart (sparsed) datels to reduce crosstalk. A translationstage may provide relative motion of the memory material and the lightbeams. In another embodiment, a light beam in the form of a sheet orhighly elliptical beam is used to simultaneously access large numbers ofdatels.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective or isometric view of the opticalmemory block portion of an optical memory in accordance with an aspectof the invention;

FIG. 2a is a simplified perspective or isometric view of a portion ofthe block of FIG. 1, illustrating the concept of addressing a particulardatel within the block by means of a focussed light beam for writing orerasure, and FIG. 2b is similar, illustrating addressing by means oforthogonal illumination (intersecting beams);

FIG. 3a is a simplified perspective or isometric view of a portion of amemory block similar to that of FIG. 1, with a laminated orlayered-sheet construction, FIG. 3b illustrates an upper edge of thestructure of FIG. 3a with light sources in the form of optical fiberscoupled to each layer of the structure;

FIG. 4 is a simplified block diagram of a memory system according to theinvention;

FIG. 5 is a cross-sectional elevation view of a portion of the block ofFIGS. 3a and 3b, illustrating overlap of adjacent focussed beams;

FIG. 6a is a schematic diagram of a datel addressing scheme whichsimultaneously addresses non-adjacent datels in a sparse manner, FIG. 6bis a frontal view of a spatial light modulator arranged for modulating aof sparse light beam array, and FIG. 6c is a cross-sectional view of theset of light beams of the sparse light beam array produced by themodulator of FIG. 6b, after focussing by a lens array, which beam arrayis usable with the arrangement of FIG. 6a;

FIG. 7 is a symbolic representation of a photochromic fulgide which maybe used in a memory according to the invention;

FIG. 8 is a schematic block diagram of an electronic sparse addressingsystem;

FIGS. 9a-9e are frontal views of various spatial light modulators usefulin the arrangement of FIG. 8;

FIG. 10a is a simplified perspective or isometric view of a memory blockand a pair of orthogonal WRITE light generators aiding in understandinganother embodiment of the invention, FIG. 10b is a perspective orisometric view of the intersecting light beams within the memory blockof FIG. 10a, and FIG. 10c is a plan view of the block of FIG. 10a,illustrating the intersecting light beams;

FIG. 11 is a simplified block diagram of another embodiment of theinvention similar to FIG. 4, in which an optical memory block isaddressed in a quasi-one-dimensional manner by crossed beams;

FIG. 12a is a frontal view of a quasi-one-dimensional spatial lightmodulator useful in the arrangement of FIG. 11, FIG. 12b represents across-section of a focussed light beam array pattern responsive to thespatial light modulator of FIG. 12a, and FIG. 12c represents the resultof compressing the pattern of FIG. 12b in a horizontal direction.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates memory material according to the invention, in theform of a rectangular block or parallelepiped 10 with sides or facesoriented parallel to X, Y and Z axes. As illustrated, block 10 has its"front" face 12 lying in the X-Y plane. Front face 12 is conceptuallydivided into a 5120-by-5120 element grid, each square element of whichrepresents the smallest storage element which is independentlyaddressable in the X-Y plane for storage of data bits. Also asillustrated in FIG. 1, block 10 is divided parallel to the X-Y planeinto 1000 "pages" P1, P2, P3, . . . P999, P1000, each of whichrepresents the smallest incremental region in the Z direction in whichstorage can occur. The intersection of the projection of each gridelement with each of pages 1-1000, where the hyphen represents the word"through," defines a rectangular "box" volume memory storage element (adatel), designated d_(X),Y,Z, in which one bit of data may be stored.While the datels of FIG. 1 are identified by dash-line outlines, itshould be emphasized that block 10 is a monolithic whole, withoutidentifiable internal boundaries; the datel locations arise due to themethod of addressing, described below in conjunction with FIGS. 2a and2b. The lengths of X, Y and Z sides of block 10 are about one inch, oneinch, and in the range between one and four inches, respectively.Naturally, the dimensions may be larger or smaller to increase ordecrease the storage capacity. In FIG. 1, elements d₁,1,1, d₂,1,1 andd₃,1,1 are illustrated as being those datels in the first, second andthird positions along the X axis, and d₅₁₁₉,1,1 and d₅₁₂₀,1,1 are thelast elements along the X axis. A 5120-by-5120 grid includes more thantwenty-six million elements. Also illustrated in FIG. 1 are datelsd₁,2,1, d₁,5119,1 and d₁,5120,1, which lie along the Y axis, and datelsd₁,5120,2, d₁,5120,999 and d₁,5120,1000. Datels d₅₁₂₀,1,2 andd₅₁₂₀,1,100 are also identified. Thus, one page of memory block 10 hasstorage datels sufficient for 5120 times 5120, or 26 Mbits. A memoryblock such as block 10 of FIG. 1, with 1000 pages, would include2.62×10¹⁰ datels, corresponding, at 8 bits/byte, to about 3.20 Gbytes ofstorage capacity. Block 10 of FIG. 1 is made from an optical memorymaterial, described below.

According to an aspect of the invention, the memory material includes acombination of a harmonic generation or up-conversion material whichtranslates light wavelength (frequency), a storage or memory materialwhich changes state in response to the translated light, and a readoutmaterial which provides an indication of the state of the storagematerial at its location. In a particular embodiment of the invention,the memory material is a mixture which includes a frequencyup-conversion material such as an up-conversion dye, a photochromicstorage material, and a fluorescent material such as a signal dye, whichresponds to the state of the local or adjacent photochromic material.The active materials may be associated with a carrier such as a polymer.In a more particular embodiment of the invention, the signal dyefluoresces when illuminated only when the storage material is in a firststate, and does not fluoresce, or fluoresces weakly, when illuminatedwhen the storage material is in a second state.

The storage component of the memory material is chosen to be a fulgide,for its thermal stability and relatively long data retention. Thefulgide changes state in response to ultraviolet (UV) or visible light.The lack of fluorescence of the fulgide is overcome by mixing it with afluorescent signal dye material, which, upon being addressed forreading, fluoresces only when the fulgide is in a particular state, andnot when the fulgide is in another state. The fulgide can havetwo-photon absorption when exposed to high intensity light, and isotherwise transparent. Usually, two photon absorption peaks at aparticular wavelength, which may not match the operating wavelength of ahigh speed spatial light modulator, described below. This mismatch canlimit the overall writing efficiency. The up-conversion dye is added toovercome or ameliorate this limitation. The up-conversion dye hasefficient two photon absorption at the operating wavelength of the highspeed spatial light modulator, and reemits at a wavelength region whichis very efficient for converting fulgide from one state to another, asfrom a color state to a bleach state. The addition to the up-conversiondye can thus enhance the overall writing efficiency and leads to reducedwriting energy. Preferred up-conversion materials are those based onsecond-harmonic generation or two-photon absorption-inducedfluorescence.

Frequency conversion dyes based on two-photon-absorption-inducedfluorescence, such as Coumarin 6, can be used to convert infrared lightin the 820 to 960 nm regime to visible light in the 550 nm range. Themixture of such a dye with a fulgide allows data writing to beaccomplished by irradiating the material with infrared light, wherebythe dye converts the infrared light to visible light, to thereby locallywrite the storage component of the memory material.

A first example of a memory material according to the invention includesthe photochromic fulgide compound E-Adamantylidene[1-(2,5-dimethyl-3-furyl) ethylidene] succinic anhydride, symbolicallyillustrated in FIG. 7, mixed with frequency-conversion dye in the formof the abovementioned Coumarin 6 (CAS No. 38215-36-0, a Kodak opticalproduct supplied by Eastman Fine Chemicals, Eastman Kodak Co.,Rochester, N.Y. 14650), and with a signal (READ) dye in the form of3,3'-diethyloxadicarbocyanine iodide (DODCI), in the preferredproportions described below. The mixture is prepared by dispersal in acarrier of polyvinylbutyral. In an experiment, a 35-μm-thick film of thedispersed material was applied to a glass slide, dried, and exposed toUV radiation at 366 nm, which changed the photochromic material from thecolorless form to the colored form. The resulting material, when exposedto a focussed 920 nm laser beam, decreased in color intensity in theexposed area, and resulted in an increase in intensity of the lightemitted in the 600 to 700 nm range when later excited by a light at 395nm. When the exposed area was illuminated with light at 366 nm, thewritten spot was erased, and the emitted light in the 600 to 700 nmrange as a result of illumination at 395 nm decreased.

The proportions of the ingredients of the mixture of the first example,as percentages of the total weight of the substance including thepolymer carrier, range from 0.001% to 10% of the photochromic component,with 2.8% preferred, 0.00001% to 1% of the frequency up-conversion dye,with 0.4% preferred, and 10⁻⁵ % to 3% of the signal dye component, with0.8% preferred.

A second example of a memory material is as described above for thefirst example, substituting polymethylmethacrylate polymer for thepolyvinylbutyral.

A third example of a memory material is as described above for the firstexample, substituting polyvinyl acetate for the polyvinylbutyral.

A fourth example of a memory material is as described above for thefirst example, substituting urethane acrylate ultraviolet (UV)-curablepolymer for the polyvinylbutyral.

A fifth example of a memory material is as described above for the firstexample, substituting UV-curable epoxy polymer for the polyvinylbutyral.

A sixth example of a memory material is as described above for the firstexample, substituting 1-(2,5-dimethyl-3-furyl)ethylidene(isopropylidene)succinic anhydride for the photochromic component.

A seventh example of a memory material is as described above for thefirst example, substituting 2,3-bis(2,4,5-trimethyl-3-thienyl) maleicanhydride for the photochromic component.

An eighth example of a memory material is as described above for thefirst example, substitutingcis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene for thephotochromic component.

A ninth example of a memory material is as described above for the firstexample, substituting1,2-dicyano-1,2bis(2-methylbenzothiophene-3-yl)ethene for thephotochromic component.

A tenth example of a memory material is as described above for the firstexample, substituting 2,3-bis(1,2-dimethyl-3-indolyl)maleic anhydridefor the photochromic component.

An eleventh example of a memory material is as described above for thefirst example, substituting 8-hydroxyl-1,3 6-pyrenetrisulfonic acid forthe Coumarin-6 component.

A twelfth example of a memory material is as described above for thefirst example, substituting Nile Red dye (CAS No. 7385-67-3 from Aldrichcatalog #29,839-5) for the DODCI component.

A thirteenth example of a memory material is as described above for thefirst example, substituting Pyridine-1 dye (Kodak CAS No. 87004-02-2,also known as LDS-722) for the DODCI component.

A fourteenth example of a memory material is as described above for thefirst example, substituting Pyridine-2 dye, available from Exciton, alsoknown as LDS-722, for the DODCI component.

A fifteenth example of a memory material is as described above for thefirst example, substituting 4-(dicyanomethylene)-2-methyl-6-(p-dimethylamino styryl)-4 H-pyran for the DODCIcomponent.

A sixteenth example of a memory material is as described above for thefirst example, substituting2[4-(4-dimethylaminophenyl)-1,3-butadienyl]-3-ethylbenzothiazoliump-toluenesulfonate for the DODCI component.

A seventeenth example of a memory material is as described above for thefirst example, substituting1,1',3,3,3',3'-hexamethyl-4,4',5,5'-dibenzo-2,2',indotricarbocyanineperchlorate for the DODCI component.

An eighteenth example of a memory material was prepared with a slice oflithium iodate (LiIO₃) crystal, cut to efficiently double the frequencyof a 920 nm laser beam. A thin film was placed on the crystal. The filmwas of polyvinylbutyral containing the photochromic compoundE-Adamantylidene [1-(2,5-dimethyl-3-furyl) ethylidene] succinicanhydride (the same photochromic compound as in the first example) inthe colored state, in a mixture with the fluorescent signal dyePyridene-1. Exposure of the crystal and the thin film to an intensefocussed 920 nm laser beam resulted in generation of 460 nm light fromthe LiIO₃ film at the focussed sites, which changed the colored form ofthe photochromic compound to its colorless form at only those sites, andwhich resulted, when read by exposure to a beam of light at 395 nm, inemission of light signal at 620-650 nm from the Pyridene-1 component ofthe memory material.

A nineteenth example of a memory material is as described above for theeighteenth example, substituting potassium dihydrogen phosphate (KDP) orpolymethyl methacrylate (PMMA) doped with 2-methyl-4-nitroanylene (MNA),or polymethyl methacrylate (PMMA) doped with para-nitroanilene (p-NA)for the LiIO₃ material. The potassium dihydrogen phosphate may becrystalline.

A twentieth example of a memory material includes a lithium iodatecrystal as described above in conjunction with the eighteenth example,substituting any of the memory materials of examples one throughseventeen for the thin film of example eighteen, with the soledifference that the memory material of the thin film usesE-Adamantylidene [1-(2,5-dimethyl-3-furyl) ethylidene] succinicanhydride and Pyridene-1.

A twenty-first example of a memory material is as described in any ofthe examples above, in which the frequency conversion material is asecond harmonic generating polymer, such as polymethyl methacrylate(PMMA) doped with a material having a high second order nonlinearhyperpolarizability, such as 2-methyl-4 nitroanilene (MNA), which, afterdoping, is poled by an electric field to align the nonlinear moieties.

While the examples given above describe various individual compositionsfor each function of frequency conversion, memory, and signalling,mixtures of the abovementioned compositions may be used to perform anyor all of the functions.

Several approaches have been proposed in the aforementioned Hunter etal. and Esner articles based on two photon absorption processes. One isto write, read, and erase a two-dimensional array simultaneously allthrough two-photon absorption of short (less than about 10 nsecduration) laser pulses. Such methods can provide high storage density,but have the disadvantage that they require high laser power forreading, writing and erasing. The data transfer rate is limited by theaverage laser power available. According to an aspect of the invention,the data transfer rates are increased by reducing the average power,which in turn is accomplished by relying upon one-photon absorption forreading. The power for reading is low enough so that it can be providedby a continuous-wave (CW), as opposed to pulsed, source, but of courseit will be recognized that the READ light source may be turned ON andOFF as required to perform the function. According to another aspect ofthe invention, the page to be read is established by control of, orreducing the size of, the memory region illuminated by the READ source.This is possible, because the use of one-photon reading increases theread sensitivity so much, by comparison with two-photon reading, thatless written material is required to produce a useful READ signal. Thisin turn reduces the amount of memory material which must be written, andtherefore reduces the total WRITE energy required. The reduction ofWRITE energy in turn allows longer WRITE pulses to be generated by theWRITE laser (or other source), or possibly even continuous-waveoperation. A further advantage of the reduced amount of memory materialwhich must be written is that fewer molecules of the storage materialmust change state during any one store/erase cycle, and if the storagematerial is subject to fatigue, the molecules available at any one datelsite will last for a larger number of cycles.

FIG. 2a shows a simplified block diagram of a portion of block 10 ofFIG. 1, illustrating one datel, designated d₂₅₀₀,2500,500, from whichidentification the datel may be recognized as being somewhere near thecenter of block 10. In FIG. 2a, a lens illustrated as a cylindricalobject 20 focusses a beam 22 of collimated light to form a convergingbeam 24, which is focussed at datel d₂₅₀₀,2500,500. According to anaspect of the invention, the energy density of the focussed beam isinsufficient to result in two-photon absorption in the material at anydatel through which the beam passes, except at the focal point in dateld₂₅₀₀,2500,500. Thus, a datel located within the body of block 10 ofFIG. 1 may be addressed without affecting adjacent datels. Thedimensions of a single datel in the x and y directions are establishedby the intensity of the focussed light beam, by the optical spatialresolution, and possibly by the granularity of the memory material.

FIG. 2b similarly illustrates datel d₂₅₀₀,2500,500, illuminated by twointersecting beams of light from two lasers 26 and 28, which produceorthogonal beams of light 261 and 281, respectively. Lasers 26 and 28each include mirrors and partially-transparent mirrors, as known, whichessentially focus the light to form collimated beams 261 and 281.Neither of light beams 26 and 28 alone has sufficient energy density towrite the datel, but together they have sufficient energy density at theintersection of beams 26 and 28, which intersection occurs within dateld₂₅₀₀,2500,500. Thus, a particular datel within the bulk of memory block10 of FIG. 1 can be addressed by different illumination arrangements, tothe exclusion of other datels, and achieve the required energy densityfor writing without additional focussing. The beams can also be used forreading, or erasing, if desired. For reading, one of the beams of FIG.2b, such as beam 281, is used, and as it traverses the line of datelsincluding datel d₂₅₀₀,2500,500, it causes those datels which are in theexcited state to fluoresce at long wavelength (about 600 to 700 nm),which fluorescence can be detected to identify the current state of thebit stored in the datel. It should be noted that as the beam diameterbecomes greater, or the spatial resolution lesser (larger spot), thatthe minimum dimension that a particular datel may have without crosstalkbecomes larger, so that larger beam diameters or poorer spatialresolution reduces the effective data capacity of a particular memoryblock. It should also be noted that the two beams of light illustratedin FIG. 2b may be at different frequencies.

Reading of written datels may be accomplished by illuminating the cubewith a sheet of light, the plane of which is orthogonal to the directionof WRITE beam 20 of FIG. 2a. A sheet or fan-shaped READ beam may begenerated by an anamorphic lens such as a cylindrical lens. The sheet oflight may traverse block 10 of FIG. 1 from top to bottom, parallel tothe X-Y plane. In order to read datels of only one page without readingdatels of adjoining pages, the read beam must be tightly focussed in theZ direction over the one-inch by one-inch area of a page, i.e. it musthave a focal depth of one inch. However, the size of the focal spot of alens is directly related to the focal depth, and a large focal depthnecessarily results in a large focal spot. This large focal spot, whenreading over a large focal depth, requires that the datels be relativelywidely spaced in the Z direction, in order to avoid crosstalk amongadjacent pixels. For a focal depth of one inch, the READ optics willhave a focal spot diameter of about 110 μm, and this becomes the limiton the minimum page thickness.

The dimension in the Z direction of a typical datel, such as dateld₂₅₆₁,2561,500 of FIGS. 2a and 2b, thus needs to be larger than, orequal to, the larger of the focal depth of the writing/erasure optics orthe focal diameter of the reading optics, in order to avoid crosstalk inwriting, erasure, or reading. For an optics to write a 5 μm spotdiameter, the focal depth for writing is less than 20 μm. As mentionedabove, a reading optics with one-inch focal depth has focal diameter of110 μm. Therefore the smallest page size that can be used for such asystem is 110 μm. Memory block 10 must be about four inches long in thez direction to accommodate 1000 pages. For a constant size memory block,the limitation set by the focal depth thus limits the volume memorydensity.

One method to reduce the dimension of the datel d₂₅₆₁, 2561, 500 in theZ direction, so that a one-inch-long memory block can accommodate 1000pages, is to use a laminar structure which includes multiple waveguidelayers. FIG. 3a illustrates a corner of a memory block 36 similar infunction to block 10 of FIG. 1, but differing in that its constructionis laminar, consisting of laminated layers. In FIG. 3a, block 36includes a plurality of glass plates or lamina 32, such as 32₁, 32₂,32₃, . . . , each of which has deposited thereon a layer of memorymaterial, according to an aspect of the invention. Thus, glass sheet 32₁supports a layer 30₁ of memory material, glass sheet 32₂ supports alayer 30₂ of memory material, glass sheet 32₃ supports a layer 30₃ ofmemory material, and memory material layer 30₄ is supported on a sheetof glass which is not illustrated in FIG. 3a. Spacer layers, describedbelow, may also be provided. The " back" surface of each sheet of glass32 is juxtaposed with the "front" surface of the layer of memorymaterial of the next higher page of memory, separated therefrom byinactive "spacer" layers 31₁, 31₂, 31₃ . . . with relatively low indexof refraction. This construction creates a laminar arrangementconsisting essentially of alternating layers of glass and activematerial. Each layer of memory material in the arrangement of FIG. 3aconstitutes one page of memory block 36. The arrangement of memory block36 of FIG. 3a has internal boundaries defining each individual page,unlike the arrangement of memory block 10 of FIGS. 1, 2a and 2b, butblock 36 has no identifiable boundaries in the X and Y directions withina page. In order to accommodate one thousand such pages within aone-inch square block, each glass-plus-memory-material layer (and spacerlayers, if used) must have a thickness not exceeding one mil (0.001inch). Writing is accomplished in the laminar structure in a mannersimilar to that described in conjunction with FIGS. 2a and 2b, and thefocussed WRITE beam forms a spot which easily fits within the one-milpage thickness.

Reading, and possibly erasure, is accomplished in the arrangement ofFIG. 3a by applying the sheet of light to the top of one of the glasslayers 32. If an anamorphic lens is used to focus a beam of light ontothe edge of each sheet of glass, the spot size can be very small in theZ direction, because the lens needs to focus only onto the edge of theglass, and the depth of focus which is required approaches zero. Eachsheet of glass has an index of refraction greater than that of theadjacent memory material, and therefore tends to act as a lightwaveguide. Light coupled into the upper edge of a sheet of glass, suchas sheet 32₂ of FIG. 3a, will be trapped in sheet 32₂ by what may beconceived of as multiple reflections at the interfaces between glasssheet 32₂ and the adjoining layers 30₂ and 31₃, and will propagatetoward the bottom of the glass sheet.

FIG. 3b illustrates a portion of an upper edge of block 36 of FIG. 3a,illustrating another way to couple light into the top of each glasslayer 32. FIG. 3b shows a plurality of optical fibers 40 terminating onan edge of each glass sheet 32. For example, a set of four opticalfibers 40_(1a), 40_(1b), 40_(1c), and 40_(1d) terminates on glass sheet32₁, and another set of optical fibers 40_(2a), 40_(2b), and 40_(2c)terminates on glass sheet 32₂. Each set of optical fibers may originatefrom an independent source, as for example optical fibers 40_(1a),40_(1b), 40_(1c), and 40_(1d) may all originate from a single starcoupler (a multiple-port optical power divider), driven by light from asingle controllable source, while optical fibers 40_(2a), 40_(2b), and40_(2c) originate from a different star coupler, driven by a separate,independently controlled, light source.

In a light waveguide, the electromagnetic fields which carry the lightenergy are principally constrained within the dielectric medium, but an"evanescent" portion of the fields lie outside the waveguide, and cancouple to the memory medium. The light which is coupled to the upperedges of glass sheets 32 in FIG. 3b proceeds downward through the glasssheets, reflecting from the sides as a result of differences between thecoefficient of refraction of the glass and the coefficient of refractionof the adjacent material, but coupling some of the light energy into thememory material, as suggested by arrows 38. READ light may be applied toone sheet of glass, such as glass sheet 32₂, for exciting those datelsof memory material of adjacent sheet 30₂ which are in the written state,and for causing them to fluoresce.

To avoid having READ light applied to a glass sheet, such as sheet 32₂of FIG. 3b, illuminate the memory material associated with an adjacentsheet which is not intended to be read, such as sheet 30₃ of memorymaterial, each layer of memory material 30 underlies a layer 31 ofspacer material, such as layer of polymer or other material with anindex of refraction lower than that of the glass. Thus, layer 30₁ ofmemory material of FIG. 3a underlies a spacer layer 31₁, layer 30₂ ofmemory material (FIGS. 3a and 3b) underlies a spacer layer 31₂, andlayer 30₃ of memory material underlies a spacer layer 31₃. When eachglass sheet 32₁, 32₂, 32₃ . . . with its deposited layers of memorymaterial 30₁, 300₂, 30₃ . . . and spacer 31₁, 31₂, 31₃ . . . are stackedtogether or juxtaposed, each glass sheet 32 is in each case (except thelast layer) juxtaposed with a spacer layer 31. The spacer layersinteract with the relatively high index of refraction of the glasslayers in a fashion well known to those familiar with dielectricwaveguides, to limit the reading/erasure light to interact with only onememory layer. Because the evanescent light intensities decayexponentially inside the layers 31₃ and 30₂ away from the interfaces,the intensity of the electromagnetic fields decay in the spacer layers31 to a level below the intensity required to produce a significant READsignal, while having sufficient amplitude within the active memorymaterial to produce fluorescence. As a result, the light propagating inany glass sheet 30 as illustrated in FIG. 3b reaches only its associatedlayer 30 of memory material, and is preferentially rejected by theadjacent spacer layers 31. In this fashion, each layer of glass of thelaminar block 36 of FIGS. 3a and 3b, and its adjacent memory material,may be independently illuminated with light for any purpose. In aparticular embodiment, light applied in this manner is used for reading,because all the datels associated with (adjacent to) a particular layerof glass may be addressed simultaneously by light applied along the edgeof the block as described in conjunction with FIG. 3b, and a selectedone of those datels may be simultaneously illuminated by a focussed orcollimated light beam as described in conjunction with FIGS. 2 a and 2b,to thereby illuminate the selected one datel with a maximum intensity oflight, andor with light of different colors.

Ideally, the difference between the coefficients of refraction of theglass and the spacer layers would be about 0.01, which would correspondwith a reflection of the WRITE beam at each layer of the laminarstructure of magnitude 10⁻⁴, whereupon the loss of the WRITE beamtraversing 1000 layers would be about 0.1. In a particular experimentalembodiment of a laminated block with glass sheets 140 μm thick, andhaving coefficient of refraction ^(n) =1.515, and with the coefficientof refraction of the memory material according to the first examplebeing ^(n) =1.47, certain datels were written. Light was then propagatedas a "sheet" through a glass sheet of a page of memory, generally asdescribed above. The emitted light from written datels was observed asfluorescence in the 600 to 700 nm range at the datel location whilelooking in the Z direction through the material.

FIG. 4 is a simplified block diagram of a memory system according to theinvention, in which the memory is a laminated block similar to memoryblock 36 of FIGS. 3a and 3b, and is identified by the same designation.The memory material is the composition described above in conjunctionwith example 1, with the above described preferred relative amounts ofthe three active components. In FIG. 4, a light source such as a laser410 applies an infrared WRITE beam at 920 nm to a beam expander 412,which expands the beam to produce an expanded, collimated light beam414. Expanded light beam 414 is passed through a polarizing filter 416to polarize the light in the direction illustrated by arrow 418. Fromfilter 416, the WRITE beam enters a polarized beam splitter 420, and isreflected to the left, and through a quarter-wave plate 424 forpolarization rotation, to a spatial light modulator (SLM) 426. Modulator426 includes as many controlled modulation pixels as there are datels ina page of memory 36, if an entire page is to be written simultaneously,or a lesser number depending on the selected array size of data to bestored simultaneously. If the selected array size is smaller than thesize of a page, defined here as a 5120×5120 size, the selected size istermed a "paragraph". For example, a paragraph can have a size of512×512, in which case a page will contain 100 paragraphs, as describedin more detail below. A preferred spatial light modulator is a 2D GaAsSLM with a high-speed response of 100 MHz or greater; a preferredmodulator is described in copending patent application Ser. No.08/109,550,filed Aug. 20, 1993 in the name of Worchesky et al. Thepixels of modulator 426 which correspond to datels of memory 36 whichare to be written are set for reflection of light, and those pixelswhich correspond to datels which are not to be written are set forabsorption. The modulator is controlled, for example, by the writecontrol portion of a computer 437 or processor with which the memoryarrangement of FIG. 4 is associated. Thus, the WRITE beam as reflectedfrom SLM 426 is spatially modulated to correspond to the relativespatial locations of the datels of memory 36 which are to be written onone paragraph or page. The modulated WRITE light beam 428 is reflectedfrom SLM 426, back through quarter-wave plate 424 to complete itspolarization rotation, and through polarized beam splitter 420. Frombeam splitter 420, WRITE light beam 428 passes through dichroic(frequency-sensitive) mirrors 430 and 432 to a dynamic focussing systemillustrated as a block 434. Dynamic focussing system 434, under thecontrol of an address control block 438 responsive to computer 437,focusses the WRITE beam at an image plane coincident with one page ofmemory 36. The focussed WRITE beam is illustrated as 436 in FIG. 4.Thus, each pixel of the WRITE beam is simultaneously focussed on itsrespective datel in the memory material, and all the datels in theparticular paragraph or page are written simultaneously. The material ofmemory 36 is transparent to the 920 nm light, so the light can befocussed at any page within the block.

The bright 920 nm spots focussed within the datels of a particular pageof memory 36 by focussing system 434 of FIG. 4 are smaller than thedimensions of the datels, so crosstalk among adjacent datels is notexpected due to the focussed spot. Another type of crosstalk isdescribed below in conjunction with FIG. 5. At the focus, each of thebright spots reaches an intensity sufficiently great so that instead ofbeing transparent, the Coumarin 6 dye absorbs the light by two-photonabsorption, and re-radiates visible light in the 500 to 550 nm range.The reradiated light is absorbed locally, within the confines of thedatel, by the photochromic fulgide material, which converts to itscolored state. Thus, the writing operation changes the state of thephotochromic material, in effect changing it from a logic "0" to a logic"1" at the particular datel. As mentioned, all the datels of a page canbe written simultaneously to any pattern of ones and zeroes. In thewritten state, the photochromic material continues to be transparent tothe 920 nm radiation, so the writing of one page does not prevent pagesmore remote from focussing system 434 ("behind" the written page asviewed from the light source) from being written.

While crosstalk among adjacent datels due to the focussed spot is notexpected, there is another potential source of crosstalk. FIG. 5illustrates a portion of a laminated memory block, similar to that ofFIG. 3a, together with simultaneously-occurring WRITE beams at 920 nm,such as those described in conjunction with FIG. 4. In FIG. 5, threemutually adjacent datels are being written by light focussed at spots510a, 510b, and 510c in memory layer 30₃. The spots are smaller than thedimensions of the datels in which they occur, so no crosstalk occursamong the datels of layer 30₃. The light beams associated with focussedspots 510a, 510b and 510c are indicated by their outlines, and aredesignated 512a, 512b and 512c, respectively. While the focus of eachbeam 512 at 920 nm causes the power density at its correspondingfocussed spot 510 to rise to a level at which the desired nonlineareffect of two-photon absorption and 550 nm radiation to occur, the powerdensity also rises in adjacent memory layer 30₂ due to the overlap ofthe beam. Beams 512a, 512b and 512c do not overlap between the focalplane 514f and another plane 514₁. Beams 512a and 512b overlap, andbeams 512b and 512c overlap, between planes 514₁ and 514₂, and all threebeams overlap to the left of plane 514₂. The power density at anycross-section of a beam 512 decreases in proportion to the square of thedistance of the cross-section from the focal point 510, so the powerdensity of the beams at large distances from focal point 510 is expectedto be very low. Near the focal point, however, the overlap of adjacentbeams may result in achieving sufficient power density for two-photonabsorption, with the result that crosstalk may occur among datels ofadjacent pages. If beams 512 of FIG. 5 are 920 nm WRITE beams, writingmay undesirably occur at corresponding datels of pages of memory otherthan the desired page.

According to another aspect of the invention, a sparse light pattern isused, to move the beam overlap locations to such a large distance fromthe focal points that the power densities of the beams are too low toaffect the memory material except at the focal points. This isaccomplished by writing mutually adjacent datels only during differentwriting intervals, so that beams are never simultaneously focussed onmutually adjacent datels of one page. As a result, the beam overlap ismoved to a more remote location. This may be understood by referring toFIG. 5, and imagining that beam 512b is eliminated, so that only beams512a and 512c are present, focussed on semi-adjacent spots 510a and510c, respectively. In such a circumstance, the first beam overlap wouldbe the overlap occurring at plane 514₂, more distant from focal plane514f than plane 514₁. As a result of the increased distance of the firstbeam overlap from the focal plane with sparse beams, the power densityat the overlap is significantly reduced, thereby reducing the likelihoodof unwanted interaction and the resulting undesired crosstalk. Byextension of the above method, simultaneous beams might be focussed onlyon spots separated by two, five, ten or more datels, thereby moving thelight beam overlaps to very large distances from the focal plane, atwhich distances the beam power densities are so low that, even whenoverlapped, the beams cannot cause an interaction with the memorymaterial so as to create crosstalk.

According to a further aspect of the invention, the sparse light patternis used in conjunction with physical translation of the memory blockrelative to the light beams, to thereby allow all datels to be accessedby a WRITE beam. FIG. 6a illustrates an arrangement of sparse beamsinteracting with a memory block. In FIG. 6a, elements corresponding tothose of FIG. 4 are designated by like reference numerals. In FIG. 6a,WRITE beam 428 consists of spaced-apart beams 623a, 628b, . . . , 628n,generated by a spatial light modulator, such as modulator 426 of FIG. 4,having an active region pattern such as that illustrated in FIG. 6b. InFIG. 6b, the active modulating regions are illustrated as hatchedregions 624, spaced from each other by absorbent regions 626. Thus, onlynon-adjoining modulated beams 628a, 628b, . . . , 628n (FIG. 6c) areproduced. Of course, the active regions 624 illustrated in FIG. 6b maybe spaced apart by two, three, five, ten or more inactive regions,depending upon how sparse the beams are to be. Nonadjacent modulatedbeams 628a, 628b, . . . , 628n of WRITE beam 428 of FIG. 6a are appliedto a microlens array 610, which focuses at a focal plane 612. WRITE beam428 of FIG. 6a is patterned by spatial light modulator 426 of FIG. 4.The pattern is focussed by a 2-D microlens array 610 of FIG. 6a. Eachlens or lenslet of microlens array 610 has a diameter matching the pixelsize of SLM 426 of FIG. 4. The focussed spot size is smaller than thesize of a lens element of microlens array 610. Therefore, a pattern iscreated at a plane 612 which corresponds with that shown in FIG. 6c withthe bright spots 690, if any, which are generated by the current stateof modulator 426, appearing at the centers of the elements of an"invisible" lattice designated 698. The side of one cell of the latticeequals the size of one element of the microlens array. Any bright spot690 in the lattice will be separated from another bright spot in both xand y directions by at least the diameter of a microlens element. Afocussing relay lens 614 couples the sparse beams to an electrooptic ormechanical zoom lens 616, which ultimately adjusts the focal depth ofthe beams within memory block 10, 36 but causes the beams to diverge.The same pattern of light and dark focal spots is then reimaged insidethe memory block by lens 618. The size of the imaged focal spots withinthe memory material is selected to be about 1/2 of the datel dimensionin the X, Y, and Z directions. At the focal plane, imaged bright spots,if any, are separated from other spots by a multiple of the dateldimensions in both the x and y directions. The multiple can be one, assuggested by FIGS. 6b and 6c, or the multiple can be two, three, ten ormore, depending how sparse the beams are designed. Thus, no adjacentdatel will be written simultaneously. As so far described, writing maybe accomplished at sparse locations at any page within the memory. Ifspatial light modulator 426 of FIG. 4 is a 5120×5120 array, tocorrespond with the datel locations illustrated in FIG. 1, but themodulator active surface is sparse by a factor of, for example, ten, sothat the active portion is 512× 512 elements, the active 512×512 portionis termed a paragraph, as described above, and there would be, in thecase of the example, 10² =100 paragraphs per page. Access to thedifferent paragraph locations is achieved by a mechanical X-Ytranslation stage 622, coupled to memory block 10, 36, which translatesby an integer number of datels, to bring different paragraphs of memoryblock 10, 36 under the sparse beam (interstitial writing). Thedisplacement in x or y direction may be 5 μm per steps for 10 steps ineach direction. Use of a piezoelectrically driven translation stage canresult in a less than 30 μsec access time between paragraphs. Thus, allportions of the memory can be written, by selecting the appropriateparagraphs by translation by means of stage 622. As an alternative totranslation stage 622, the interstitial writing can be achieved bytilting a transparent 2 mm parallel plate 620 in a step of about 0.2°around the x, y, or both axes to provide x or y motion of the focussedbeams, or to displace the image to any desired interstitial location.

Reading is accomplished by applying a "sheet" of light to the glasssheet associated with one layer or page of memory 36 of FIG. 4, underthe control of address block 438, which is ultimately under control ofthe memory read-write portion of the associated computer. In FIG. 4, aread beam 400 at about 400 nm is applied to an acoustooptic (AO) deviceillustrated as a block 442, together with additional control signals, ifnecessary, for scanning the read beam from page to page of the memory,and the resulting beam is spread along the upper edge of the glass sheetof the appropriate page by a spreading device illustrated as acylindrical lens 444 for creating a highly elliptical beam, the majoraxis of which is parallel to one of the glass layers of the memory, forbeing coupled thereinto. Coupling of read light could also beaccomplished by a system of optical fibers and star couplers asdescribed in conjunction with FIGS. 3a and 3b. While the memory materialhas a moderate absorption cross-section at 400 nm, the transparent glassguiding layer carries the read light deep into the cube as described inconjunction with FIG. 3b. The 400 nm light is absorbed by the DODCIcomponent of the memory material, which fluoresces at a wavelengthlonger than 615 nm only from locations at which the logic one state isstored, and not from locations at which a logic zero state is stored.Thus, a region adjacent to the glass sheet fluoresces to provide anindication of the state of the memory material. Because of theevanescent decay of the 395 nm read beam light in the memory materialand in the spacer, pages of memory remote from the sheet of glassassociated with the page being read do not receive sufficient light tofluoresce. The fluorescing signal dye material produces light at awavelength longer than 615 nm, to which the memory material istransparent. The fluorescence of the DODCI or other signal dye at ornear the focal plane is picked up by focussing system 434, and formedinto a collimated beam 448, which passes through dichroic mirror 432,and reflects from dichroic mirror 430, to direct the collimated lightbeam through an array of pinholes onto an output array 450 ofphotodetectors. The array of pinholes has a sparse pattern to match theformat of the light array created at the plane 612 of FIG. 6a. Moreparticularly, each pinhole is located at the center of a cell of theinvisible sparse lattice. The diameter of each pinhole is much smallerthan the size of the corresponding cell. The collected light beam willform an image at the plane where the pinholes locate. Only those passingthrough the pinholes will are reimaged by the pinholes, to produce adiverging beam from each pinhole which is illuminated by the sparsepattern. The diverging beam from each pinhole intercepts a photodetectorin the plane of the array of photodetectors. Array 450 includes onelocation or pixel for each datel of one paragraph or page of memory 36.Light beam 448 is sensed by array 450, and only those pixels respondwhich are associated with fluorescing ones of the datels of memory 36,which means that the pixels of detector array 450 which respond arethose which receive fluorescent light from datels of memory 36 at whicha logic one was stored. Since datels of memory 36 at which a logic zerowas stored do not fluoresce when illuminated by a read beam, thosepixels of detector array 450 which correspond to the zero-storing datelsdo not respond. Thus, the pattern of ones and zeroes stored in oneparagraph of one page of memory 36 is replicated on detector array 450when the whole paragraph or page is addressed by a read beam. Array 450may be, for example, a CCD photosensor array, well known in thetelevision art. The image-representative signals may be read in aconventional manner with a parallel output signal bus, and coupled to autilization apparatus such as a computer.

As mentioned above, the use of one-photon absorption signal dye, such asDODCI, for reading, reduces the power requirements of the READ lightsource or laser, and because the one-photon material is more efficientin producing fluorescence, reduces the amount of memory material whichmust be written in order to produce a discernible READ signal. Thereduced requirement for written material also reduces the powerrequirements of the WRITE and ERASE light sources. An analysis of systempower requirements of 2-D read, write and erase systems which usetwo-photon absorption processes exclusively suggests that such systemsrequire high power, short-pulse lasers for reading, whereas the systemsof the invention require significantly less laser power, which may beavailable from a CW laser. Also, the inventive systems appear to requireabout one-tenth the pulsed power for writing and erasure, and one-thirdthe reading power. The reduced power requirements of the systemaccording to the invention at least allows a greater pulse repetitionrate for reading and writing by comparison withexclusively-two-photon-absorption, so that an improvement of datatransfer rate by a factor of 10 is anticipated.

As mentioned, one of the advantages of the use of fulgides as memorymaterials is that they are relatively stable, and can maintain thewritten state for years at room temperature. However, erasure may oftenbe desired in ordinary operation of a data memory. In the arrangement ofFIG. 4, erasure is accomplished by applying an intense beam of light at710 nm to the datels to be erased. More specifically, the computerselects those datels which are to be erased at each page or currentparagraph of the current paragraph or page, if applicable, of memory.For each page (or paragraph thereof), those pixels of a spatial lightmodulator 460 corresponding to the datels to be erased in the currentparagraph or page are set to a reflective condition. A collimated,unmodulated ERASE light beam, illustrated as 462, is applied through apolarizing filter 463, and the polarized light reflects from a polarizedbeam splitter 464, and passes through a quarter-wave plate, generally asdescribed above in relation to the WRITE beam. Portions of the ERASEbeam arriving at spatial light modulator 460 are reflected, and thereflected portions, corresponding to the datels to be erased, pass onceagain through the quarter wave plate, and through beam splitter 466, toform a modulated ERASE beam designated 470. ERASE beam 470 is reflectedby mirror 472, and by dichroic mirror 432, to pass through focussingsystem 434. The memory material is transparent to the 710 nm light, soit can reach any location within block 36. The photochromic component ofthe memory material absorbs the focussed 710 nm light at the focal planethrough a two-photon process, and switches to its ground state. Thus,the memory material can be erased. Those datels of memory which liewithin one page of memory, and which are to be erased, can be erasedsimultaneously.

It may be desirable to operate the memory arrangement without mechanicaltranslation of the memory block or the light beam source. In FIG. 8, asource of data 810 produces parallel data to be stored, together withclock and timing signals, which are applied to a one-of-four multiplexer812. One-of-four multiplexer 812 accepts the parallel data to be stored,and routes the first set of data (one paragraph) to a spatial lightmodulator (SLM) 826a. Light modulator 826a is associated with apolarized beam splitter 420a and a quarter-wave plate 824a formodulating an unmodulated collimated, polarized WRITE beam 814a, toproduce a modulated output beam 828a. FIG. 9a illustrates a portion ofthe face of spatial light modulator 826a, identifying the activeelements by the numeral "1". Each active element modulates one bit ofthe data to be stored. The row and column locations of the activeelements are identified by roman numerals and capital letters,respectively. As illustrated, the active elements of SLM 826a are IA,IE, IIC, IIIA, IIIE, IVC . . . The other elements of SLM 826a areabsorptive, and do not modulate the beam. One-of-four multiplexer 812routes the second set of data from source 810 of FIG. 8 to an SLM 826b,which is associated with a polarized beam splitter 420b, and aquarter-wave plate 824b for modulating an unmodulated WRITE beam 814b,to produce a modulated output light beam 828b. FIG. 9b illustrates aportion of the face of SLM 826b, corresponding to the portion of SLM826a illustrated in FIG. 9a, with the active elements designated by thenumeral "2". The third and fourth sets of data from source of data 812of FIG. 8 are routed to SLMs 826c and 826d, which are associated withquarter-wave plates 824c and d and polarized beam splitters 420c and420d as described above, for modulating beams 814c and 814d,respectively. The unmarked elements are absorptive. FIGS. 9c and 9drepresent by the numerals "3" and "4", respectively, the active elementsof the illustrated portion of the faces of SLMs 826c and 826d,respectively.

As illustrated in FIGS. 9a-9d, no numeral is adjacent a like numeral, soat any one time, the modulation is sparse. Referring to FIG. 8, a mirror840 reflects modulated light beam 828a toward half-silvered mirror 842a,where light beam 828a would combine with beam 828b, if both existedsimultaneously. The "two" beams proceed toward half-silvered mirror842b, where they "combine" with beam 828c. All the beams are "combined"together by half-mirror 842c. When properly aligned so that the elementsare in registry, the beams generated by the various active elements areinterspersed, as illustrated in FIG. 9e. The interspersed beams areapplied through a dynamic focus arrangement 434 similar to thatdescribed above, for focussing into memory block 10 or 36. To reduceloss, polarized beam combiners could be used instead of half-silveredmirrors.

In operation of the arrangement of FIG. 8, the combined WRITE beam issequentially modulated by the patterns of FIGS. 9a, 9b, 9c and 9d.Therefore, the focussed light beams within the memory block are at alltimes spaced apart, but writing to the various locations of differentparagraphs is accomplished without physical translation of the memoryblock.

If the combination of a plurality of modulators as illustrated in FIG. 8is insufficient in combination to write one entire page of memory, asfor example if each of the four SLMs 726a-d is a 502×502 modulator, anda page as 5020×5020 datels, the electronic sparse writing scheme may beused in combination with a translation arrangement, with the advantageof reducing the number of translations per unit time, and increasing theminimum step size.

The arrangement of four discrete SLMs arranged as in FIG. 8 may beviewed as a single SLM, in which all elements are active as in FIG. 9e,but in which the data is applied to different sets (sets 1, 2, 3 4) ofthe elements, and the other elements remain in their absorptive state.Thus, a simple large SLM may be used, in which all elements are active,but which are sequentially enabled in a sparse manner, as suggested byFIG. 9e. When a large number of inactive modulator elements separate theactive elements, a very large number of possible sparsing patternsexist.

FIG. 10a is a simplified perspective or isometric view useful inexplaining another embodiment of the invention. In FIG. 10a, a block 10of optical memory material, which may be a block using the fulgide andsignal dye compositions described above, but without the upconversiondye, is associated with a first WRITE light projector 1010, whichincludes a source of light 1012, a beam expander 1014, if necessary, anda cylindrical lens 1016 for shaping the expanded light into a thin sheetof light or "fan" shaped beam 1018, elongated in the Y direction, whichimpinges on face 12 of memory block 10. The plane of sheet beam 1018 isparallel to the YZ plane. A second WRITE light projector 1020 includes asource 1022 of light, a beam expander if necessary, and a cylindricallens 1026, for shaping the light from source 1022 into a thin fan beam1026, elongated in the Y direction, impinging upon block 10 at a faceorthogonal to face 12. The sheet of light beam 1026 lies parallel to theXY plane. Fan beams 1018 and 1026 intersect within the block along avertically disposed (parallel to the Y axis) line or column, illustratedas 1030 in FIG. 10b and in the top view of FIG. 10c. With thearrangement of FIGS. 10a, 10b, and 10c, a vertical column of memorymaterial may be written simultaneously, with the transverse dimensionsof the column being established by the dimensions of the intersection ofthe beams. The two fan beams will tend to have relatively largetransverse or lateral dimensions near the cylindrical lens by which theyare focussed, and also far from the focal point, as suggested by theshape of beams 1018 and 1026 in FIGS. 10b and 10c, with a narrow orfocussed region (a waist) at moderate distances from the cylindricallens. The two beams are made to intersect at a location such as 1030near the focus of the beams, to reduce the size of the column of datelswhich is ultimately written at the beam intersection. The beamintersection will have finite dimensions, but writing does not takeplace at any location within the memory block, except at theintersection, because that is the only location at which the beamintensity is high enough to cause the nonlinear interactions whichresult in writing. The memory material may be a two-photon absorbingmaterial, such as those described above. The two beams may be atdifferent frequencies, such as 1300 nm and at 890 nm, for writing to theabovementioned fulgide materials, but the frequencies may of course beselected for the particular material being used.

As so far described, the arrangement of FIGS. 10a, 10b, and 10c iscapable of writing only to a narrow cylindrical volume (1030 of FIGS.10b and 10c) vertically disposed (parallel to the Y axis) in the memorymaterial. If, however, light source 1012 of FIG. 10a includes amodulator, individual datels may be simultaneously written into aparticular column. Recognizing that the memory material contains nointernal boundaries, it is useful to identify locations by their column,and by the "floor" or "story" in that column. If, for example, WRITElight source 1012 of FIG. 10a includes a column light modulator, such asa vertical 1×1024 pixel array, with one pixel above another, as many as1024 pixels could be simultaneously written at different verticallydisposed stories in column volume 1030 of FIGS. 10b and 10c. Each story,in that instance, would contain one datel. Thus, a "paragraph" asdescribed in conjunction with the arrangement of FIG. 4 corresponds, inthe embodiment of FIG. 10a, to a column. The lateral dimensions of thecolumn are expected to be sufficiently large so that writing singledatels into each story would be wasteful of useful memory material.

According to another aspect of the invention, light source 1012 of FIG.10a includes a two-dimensional column modulator, such as an 8×1024modulator. While it is actually a two-dimensional modulator, itsdimensions are so much like a column that it may be considered to bequasi-one-dimensional (q-1-D). Thus, this approach may be termed"q-1-D." FIG. 11 is a simplified block diagram of a memory systemaccording to the q-1-D approach, and is generally similar to FIG. 4. InFIG. 11, elements corresponding to those of FIG. 4 are designated bylike reference numerals in the 1100 series rather than the 400 series.In FIG. 11, an unmodulated WRITE light source 1110 at 890 nm is appliedto a 1-D or line beam expander 1112, which generates unmodulated 1-DWRITE beam 1114. A polarizer 1116 polarizes beam 1114 in the directionof arrowhead 1118, and the resulting polarized beam is reflected bypolarized beam splitter 1120, to form beam 1122. Beam 1122 is appliedthrough a quarter-wave plate 1124 for polarization rotation, and isapplied to a q-1-D (8×1024 pixel) spatial light modulator 1126, to whichthe information to be written to the memory cube is also applied fromcomputer 1137. The spatial light modulator lies parallel to line beam1122, and the pixels are modulated by the information, to produce a 1-DWRITE beam, modulated by reflection of the associated pixels of the SLM.The modulated beam passes through quarter-wave plate 1124 to completeits polarization rotation, emerging as a beam 1128. Beam 1128 passesthrough beam splitter 1120, and through mirrors 1132 and 1148, andthrough a focussing system 1134 which may include a zoom lens controlledby address manager 1138. From focussing system 1134, the modulated 1-Dbeam passes through an acoustooptic device controlled by address manager1138, for being deflected to cause beam 1128 to traverse the column tobe written, and the deflected modulated beam 1136 is refocussed by alens system 1190 as a sheet within cube 10. Lens system 1190 in effectadjusts the focus of beam 1136 to place the waist of the beam near thedesired intersection column 1030 within the cube. However, the modulated890 nm WRITE beam 1136 does not have the intensity at any locationwithin cube 10 to write datels.

At the same time that modulated beam 1136 is applied to cube 10, asecond unmodulated sheet WRITE beam 1146 at 1300 nm is applied from anorthogonal direction, as generally described in conjunction with FIGS.10a, 10b, and 10c. Beam 1146 of FIG. 11 originates as beam 1101 from asource (not illustrated in FIG. 11), is reflected from dichroic mirror1196, and passes through dichroic mirror 1198 to a dynamic focussingsystem 1194 and acoustooptic device 1142, both controlled by addressmanager 1138, which adjust the focus of beam 1146 to place the waist ofbeam near the desired intersection point within the cube. Beam 1146alone also lacks sufficient energy to cause writing, but at theintersection column 1030, the combined energies are sufficient to writeat the bright spots resulting from the reflective pixels of spatiallight modulator 1126. Unmodulated WRITE beam 1146 may be viewed as"sensitizing" a plane of memory material, so that writing may beaccomplished in the sensitized plane by the bright spots of intersectingmodulated WRITE beam 1128.

The sparsing of the beams described in conjunction with FIGS. 5, 6a and6b is for the purpose of prevention of writing to pages of the memorymaterial which are closer to and farther from the WRITE beam source(adjacent pages in the Z direction) than the page to which data is to bewritten. The possibility of writing to such adjacent pages, in turn,arose from the high power density of the WRITE beam, and from thepossibility of overlapping of the beams of adjacent datels beingwritten. In the embodiment of FIGS. 10a, 10b, 10c, and 11, thepossibility of crosstalk to adjacent pages does not exist, becauseneither of the two intersecting beams alone has enough power density towrite, and, so long as the waist region of unmodulated sheet WRITE beam1146 is sufficiently small in the intersection region, it "sensitizes"only one page, and there is no possibility of writing to pages which areadjacent in the Z direction. Consequently, the modulated WRITE beamsneed not be sparse in the X direction. However, crosstalk between datelscan take place in the Y direction, because the unmodulated WRITE beamsensitizes an area, rather than a line, in the XY plane. Therefore, theWRITE beam must be sparse in the Y direction.

FIG. 12a is a view of a portion of 8×1024 spatial light modulator array1126 of FIG. 11, in which there are eight pixels horizontally, and 1024pixels vertically. Shaded pixels, such as pixels 1210, are absorptive,and modulate the write light beam with a "dark" spot of zero intensity(no writing), while the unshaded pixels, such as pixels 1212, representreflective pixels which produce bright spots when the beams arefocussed. FIG. 12b is an illustration of the bright spots (if any existas a result of the information being modulated) of the "invisiblelattice", which occur at a focal plane in the optical system of FIG. 11corresponding to plane 612 of FIG. 6. In FIG. 12b, dots 1210 representthe bright spots, which correspond with reflective pixels 1214 of FIG.12a. FIG. 12c illustrates the result of passing the light pattern ofFIG. 12b through cylindrical lens 1190, which results in compression ofthe pattern of FIG. 12b in the direction of arrows 1200 of FIG. 12b,i.e. reduces the X dimension without reducing the Y dimension. In FIG.12c, all the spots 1222 are represented as bright spots for ease ofunderstanding. The eight spots 1222 of each horizontal row are closertogether in the X or horizontal direction than in FIG. 12b, but notcloser in the Y or vertical direction. This is the definition of a beamarray which is sparsed in the vertical direction. Thus, the beamproduced by WRITE light source 1110 of FIG. 11 is made into a quasi-linebeam by expander 1112, the beam is modulated, and the modulated beam isagain passed through a cylindrical lens to reduce it to the final q-1-Dform, thereby accomplishing the sparsing in the vertical direction, asexplained in conjunction with FIG. 12a, 12b and 12c.

Erasing is accomplished in the arrangement of FIG. 11 in generally thesame manner as writing. In FIG. 11, an erasure beam 1162 at 710 nm isapplied through a polarizer 1164, and the polarized beam is reflected bya polarized beam splitter 1166, through a quarter-wave plate to an erasespatial light modulator similar to SLM 1126, which receives informationfrom a source (not illustrated) relating to the datels to be erased. Themodulated ERASE beam, represented as 1170 is reflected by a mirror 1172and by dichroic mirror 1132, and then follows the same path as thatdescribed for the unmodulated WRITE beam through focussing system 1134,acoustooptic device 1188, and lens system 1190. Thus, a sheet ERASE beammodulated with the datels to be erased is applied to memory cube 10, ata power level insufficient to erase. At the same time, a second,unmodulated ERASE beam 1186 is reflected by dichroic mirror 1196, andfollows the same path therefrom as the unmodulated READ beam 1100,through focussing system 1194, acoustooptic device 1142, and lens system1192, into cube 10. The modulated and unmodulated ERASE beams intersectin a column, with bright spots at the locations to be erased, much asdescribed in conjunction with the READ function. Alternatively, ifentire columns are to be erased simultaneously, erasure beam 1162 can beapplied directly to mirror 1172, without passing through SLM 1160.

Reading is accomplished in the arrangement of FIG. 11 in a mannergenerally similar to that described in conjunction with FIG. 4, in thata planar light beam is passed through the block of memory material, tocause one-photon absorption by the signal component of the memorymaterial. In FIG. 11, a READ light beam 1100 at 395 nm from a source(not illustrated) is applied through dichroic mirrors 1196 and 1198, andthereafter the READ light beam follows the same path as the abovedescribed erasure beam 1186, being converted to a sheet or fan beam witha narrow waist, such as that described in conjunction with FIGS. 10b and10c, near the region to be read. The sheet READ beam is deflected by anacoustooptic device 1192 to the appropriate Z position to intercept thecolumn to be read. In the region illuminated by READ beam 1100 withincube 10, the signal dye associated with datels in a written statefluoresces. The fluorescence of a selected q-1-D column is imaged bylens system 1190, acoustooptic device 1189, and focussing system 1134,to form a read information beam 1148, which is coupled to an outputarray 1150 of detectors by dichroic mirrors 1130 and 1132. Detectorarray 1150 responds to the information light by producing parallelsignals representing the state of each of the datels of q-1-D column1030 which has been read. Crosstalk is avoided during reading because ofthe narrow waist of READ beam 1100 within the cube, whereby the memorymaterial tends to respond at only one page, and by the narrow depth offocus of focussing system 1134, AO device 1189, and lens system 1190.

Since the positions of the beam intersections 1030 within cube 10 ofFIG. 11 are established by beam deflection provided by acoustoopticdevices 1142 and 1189, physical translation of the cube relative to thebeam-forming structure, such as that described in conjunction with FIG.4, is unnecessary.

Other embodiments of the invention will be apparent to those skilled inthe art. For example, a plurality of light sources such as laser diodesor LEDs may be coupled to the upper edges of glass sheets 32 of block 36of FIG. 3b instead of using optical fibers 40. While the memory materialhas been described as being dissolved in a polymer which is thensolidified, there is no theoretical necessity for the active memorymaterial to be in the solid form; it could as easily be dissolved in afluid such as a liquid which is contained in a transparent cellularstructure, in which each cell constitutes a datel. Thus, when thematerial is a fluid, a "slab" requires a restraining boundary. Whilemechanical translation of the memory block relative to the beams hasbeen described in conjunction with FIGS. 6a and 6b, the light beamsthemselves can be translated by translating their effective sourcerelative to the memory block, or both could be translated. While aparagraph/page system has been described, there is no requirement inprinciple that pages be subdivided. Instead of using a reflectivespatial light modulator together with a polarized beamsplitter, asdescribed, a WRITE laser array with individually controllable driverscould be used. While the sparse addressing scheme has been described asbeing for the WRITE function, it may be used, if desired, for either orboth of READ and ERASE functions. While the light sources have beendescribed as being lasers, other light sources with equivalentcharacteristics may be used. The signal dye, if used in the memorymaterial, may respond when associated either with written or unwrittenstorage component of the memory material. The addressing arrangementsaccording to the invention may be used with memory materials accordingto the invention or with other memory materials, and memory materialsaccording to the invention may be used with other addressingarrangements.

What is claimed is:
 1. An optical memory material, comprising:a fulgidestorage component which converts from a first state to a second state inresponse to two-photon absorption of WRITE light; and a signal componentwhich responds to one-photon absorption by fluorescing only at locationsat which said storage component is in one of said first and secondstates, and not in the other one of said first and second states.
 2. Amaterial according to claim 1, wherein said signal component respondsonly at locations at which said storage component is in said secondstate.
 3. A material according to claim 1, wherein said signal componentconsists essentially of one of (a) 3,3'-diethyloxadicarbocyanine iodide,(b) Nile red dye, (c) Pyridine-1, (d) Pyridine-2, (e) 4-(dicyanomethylene)-2-methyl-6-(p-dimethylamino styryl)-4 H-pyran, (f)2[4-(4-dimethylaminophenyl)-1,3-butadienyl]-3-ethylbenzothiazoliump-toluenesulfonate, (g)1,1',3,3,3',3'-hexamethyl-4,4',5,5'-dibenzo-2,2',indotricarbocyanineperchlorate, and (h) mixtures thereof.
 4. A material according to claim1, further comprising an associated apparatus, together forming asystem, said associated apparatus comprising:means for concentratingwriting light onto at least a portion of said material, for causing saidmultistate storage material to absorb said photons of light bytwo-photon absorption, for changing said storage component from saidfirst state to said second state; and means for irradiating at least aportion of said active materials with reading light, for causing saidsignal component to respond, except at those locations at which saidstorage component is in said one of said first and second states.
 5. Asystem according to claim 4, wherein said means for concentratingcomprises one of focussing means and crossed beam means.
 6. A memorymaterial according to claim 1, comprising:a frequency upconversioncomponent which, upon receipt of a beam of light, converts at least aportion of said beam of light to a higher frequency by two-photonabsorption, to thereby generate at least a portion of said WRITE light.7. A material according to claim 1, wherein said storage componentconsists essentially of one of (a) E-Adamantylidene[1-(2,5-dimethyl-3-furyl)ethylidene]succinic anhydride, (b)1-(2,5-dimethyl-3-furyl) ethylidene (isopropylidene) succinic anhydride,(c) 2,3-bis(2,4,5-trimethyl-3-thienyl) maleic anhydride, (d)cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene, (e)1,2-dicyano-1,2bis(2-methylbenzothiophene-3-yl)ethene, (f)2,3-bis(1,2-dimethyl-3-indolyl)maleic anhydride, and (g) mixturesthereof.
 8. An optical memory material comprising:a fulgide storagecomponent which converts from a first state to a second state inresponse to two-photon absorption of WRITE light; mixed with a signalcomponent which responds to one-photon absorption by fluorescing only atlocations at which said storage component is in one of said first andsecond states, and not in the other one of said first and second states.9. A material according to claim 8, wherein said storage componentconsists essentially of one of (a) E-Adamantylidene[1-(2,5-dimethyl-3-furyl)ethylidene]succinic anhydride, (b)1-(2,5-dimethyl-3-furyl) ethylidene (isopropylidene) succinic anhydride,(c) 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride, (d)cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene, (e)1,2-dicyano-1,2bis(2-methylbenzothiophene-3-yl)ethene, (f)2,3-bis(1,2-dimethyl-3-indolyl)maleic anhydride, and (g) mixturesthereof.
 10. A memory material according to claim 8, wherein said signalcomponent consists essentially of one of (a)3,3'-diethyloxadicarbocyanine iodide, (b) Nile red dye, (c) Pyridine-1,(d) Pyridine-2, (e) 4-(dicyano methylene)-2-methyl-6-(p-dimethylaminostyryl)-4 H-pyran, (f)2[4-(4-dimethylaminophenyl)-1,3-butadienyl]-3-ethylbenzothiazoliump-toluenesulfonate, (g)1,1',3,3,3',3'-hexamethyl-4,4',5,5'-dibenzo-2,2',indotricarbocyanineperchlorate, and (h) mixtures thereof.
 11. A material according to claim8, further comprising an associated apparatus, together forming asystem, said associated apparatus comprising:means for concentratingwriting light onto at least a portion of said material, for causing saidfulgide storage component to absorb said photons of light by two-photonabsorption, for changing said fulgide storage component from said firststate to said second state; and means for irradiating at least a portionof said optical memory material with reading light, for causing saidsignal component to respond, except at those locations at which saidstorage component is in said one of said first and second states.